1. Field of the Invention
The present invention relates to the field of patterning indium-tin oxide films, and more particularly, to the patterning of such films by plasma etching.
2. Background Information
Indium-tin oxide, also known as ITO, is widely used in the electronics industry as a transparent electrode. It has particular application in liquid crystal displays where one of the electrodes which controls the state of the display is disposed between the viewer and the liquid crystal material. It is also used where a transparent electrode is needed in a solid state imager where that electrode is disposed between the light sensitive portion of the structure and the source of the light.
It is known that indium-tin oxide can be wet etched using hydrochloric acid, however, such etching of indium-tin oxide is not widely used because the HCl tends to undercut the photoresist mask pattern while it is etching the indium-tin oxide with the result that poor resolution is obtained.
It is also known to etch indium-tin oxide films in a plasma containing chlorine ions or other halogenated ions. This process does not suffer from the undercutting and occasional residual conductive spots which are characteristic of wet etching processes for etching indium-tin oxide.
It is well known in the electronics arts that etching processes which are used to pattern a layer on top of a substrate, must either not etch the substrate material or must etch the substrate material at a much slower rate than they etch the material being patterned. This is in order to ensure that the etching process can be reliably stopped at a point where the etching of the material being patterned has gone to completion, and the substrate is still unaffected or has been only minimally affected by the etching process. An etchant which has this characteristic is referred to as a selective etchant. It is generally accepted that, for etch stop purposes, a selective etchant must etch the material it is desired to remove at least five times faster than it etches the underlying material it is desired to leave in place. A preferred selective etchant is one which is selective for the material to be patterned and against all other materials which it may be desired to have underneath that material-to-be-patterned.
Indium-tin oxide, in electronic applications, typically is disposed on a substrate whose exposed surface includes one or more of silicon, molybdenum, aluminum, titanium, silicon nitride, silicon oxide (typically silicon dioxide), which it is desired to retain in the final structure. Thus, to be truly selective in electronics applications, an etchant for indium-tin oxide would have to leave all of these materials essentially undisturbed. In a HCl plasma, silicon etches essentially ten times faster than indium-tin oxide; molybdenum etches more than six times faster than indium-tin oxide; aluminum etches at essentially two-thirds the rate of indium-tin oxide; silicon oxide and silicon nitride each etch at substantially one-half the rate at which indium-tin oxide etches and titanium etches at substantially one-third the rate at which indium-tin oxide etches. Use of other halogenated etchants results in similar non-selective etching of indium-tin oxide. Even when a halogenated etchant contains a source of organic radicals, the much faster etch rates of the halogenated compounds predominate and substantially no etching results from the interaction of organic radicals and the metal being etched.
Accordingly, plasma etching of indium-tin oxide using halogenated gases is not a desirable process in the electronics arts where any one of these materials underlies the indium-tin oxide.
It has been reported in the literature that indium and tin can be individually etched in discharges containing methane or acetone. In each of these cases, the effective etching species is assumed to be the methyl radical. Unfortunately, this process results in the deposition of carbon containing films, typically polymers, on all exposed surfaces in the reactor which do not etch in the discharge. This is undesirable from a number of viewpoints. First, it results in the formation of undesired layers on the remainder of the component being etched. Such additional layers would have to be removed as part of the process of forming the device being produced. Second, the formation of these films on the surface of the reactor is undesirable because of the potential to contaminate other devices being etched in the reactor with other plasma compositions. Third, film deposition on the chamber surfaces necessitates frequent cleaning, leading to a loss of productivity. Because of these etching characteristics, this chemistry is not used in the electronics arts to pattern indium-tin oxide.
As a consequence, an alternative process known as lift-off is used to pattern indium-tin oxide films. In the lift-off process, a photoresist layer is deposited on the substrate and patterned with a re-entrant profile to expose the underlying substrate in those locations where the indium-tin oxide film is desired. The indium-tin oxide film is then deposited over that structure, which includes the exposed portions of the substrate and the remaining photoresist. After the desired thickness of the indium-tin oxide film has been deposited, the remaining photoresist is removed from the substrate using a solvent in which the photoresist is soluble, leaving in place that portion of the indium-tin oxide which was deposited directly on the substrate surface. While this lift-off process is generally effective for small areas, it becomes cumbersome and less reliable where large areas of patterned ITO film are required as in large area liquid crystal displays. This is because the photoresist dissolves at the edge of the substrate and then progressively inward. This lift-off process also has a disadvantage in that the presence of the photoresist prevents the use of an effective back sputtering immediately before ITO deposition to clean the substrate surface to remove incidental surface films such as air reaction oxides, oils or other low concentration contaminants which increase the resistance of the ITO-to-substrate contacts.
For large area structures and where finer detail is also required, an improved process for patterning indium-tin oxide is needed.